A related art light wave radar apparatus is provided with a light source for emitting a light signal, a light intensity modulator for modulating the light signal emitted from the light source to output pulsed light, a light amplifier for amplifying the pulsed light modulated by the light intensity modulator, a light transmit-receive system for emitting the pulsed light amplified by the light amplifier toward a space, and for collecting scattered light resulting from a scattering of the pulsed light by the space, and a receiving circuit for Doppler radar for heterodyne-detecting both a local part of the light signal emitted from the light source, and the scattered light collected by the light transmission-receive system so as to acquire the Doppler frequency of the wind velocity in the space (for example, refer to patent reference 1).
Such a related art light wave radar apparatus disposed in an airplane can cancel the Doppler frequency of the flying speed of the airplane so as to acquire the Doppler frequency of the wind velocity.
However, when the pulsed light amplified by the light amplifier is made to propagate to the light transmission-receive system by way of a waveguide of optical-fiber type, a change occurs in the refractive index of the waveguide in proportion to a change in the intensity of the pulsed light according to the nonlinear effect of the fiber core medium. As this refractive index change occurs, the phase of the pulsed light propagating changes (self-phase modulation). It is known that this phase change is proportional to the intensity of the pulsed light (Kerr effect).
Furthermore, since the intensity of the pulsed light changes with time, the phase of the pulsed light also changes with time. Since a rate of change in the phase of light which changes with time corresponds to a change in the frequency of the light, the frequency of the pulsed light transmitted deviates from its original value.
In addition, when the frequency of the pulsed light transmitted deviates from its original value, an offset occurs in the Doppler speed (equivalent to the Doppler frequency) of the wind velocity which is measured by the receiving circuit for Doppler radar.
For example, in a case where the related art light wave radar apparatus transmits the pulsed light of a wavelength of 1.5 μm, when a frequency deviation of 1.3 MHz occurs in the pulsed light, the frequency deviation corresponds to an offset error of 1 m/s in the Doppler speed.
[Patent reference 1] JP,2003-240852,A (see paragraphs [0016] and [0024], and FIG. 1)
A problem with the related art light wave radar apparatus constructed as mentioned above is that when the frequency of the pulsed light transmitted deviates from its original value, an offset occurs in the Doppler speed of the wind velocity which is measured by the receiving circuit for Doppler radar, and therefore the wind velocity in the sight line direction cannot be measured with a high degree of precision.
The present invention is made in order to solve the above-mentioned problem, and it is therefore an object of the present invention to provide a light wave radar apparatus which detects a frequency deviation of pulsed light transmitted to outside to carry out a high-precision measurement of a wind velocity.